Non-carcinogenic, non-polluting sulfuric acid etchants for preparing aluminum surfaces for adhesive bonding

ABSTRACT

The invention provides a process and composition of low toxicity for  prepng aluminum surfaces for adhesive bonding. According to the invention non-carcinogenic, non-polluting soluble metal salts generally, other than ferric sulfate disclosed in U.S. Pat. No. 4,212,701, when added in an effective amount to sulfuric acid, improve the adhesive bonding to aluminum when the aluminum is treated with such salt-sulfuric acid etchants prior to adhesive bonding.

GOVERNMENTAL INTEREST

The invention described herein may be manufactured, used and licensed byor for the Government for Governmental purposes without payment to us ofany royalties thereon.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,212,701 discloses an improved procedure for preparingaluminum surfaces for adhesive bonding, which comprises etching thealuminum surface with an etching bath consisting essentially of amixture of aqueous sulfuric acid and ferric sulfate. The procedureeliminates the pollution problem and toxic and carcinogenic fumesassociated with the conventional procedures, wherein the aluminum isetched with an aqueous solution of sulfuric acid and sodium dichromate;and it produces aluminum surfaces, which when adhesively bonded, providejoint strengths comparable to those obtained using the standard sulfuricacid-sodium dichromate etch. In a further study of the process of U.S.Pat. No. 4,212,701, it was suggested that the ferric ion in the ferricsulfate-sulfuric acid etchant for aluminum functions by both retardingsulfuric acid attack on the surface and by oxidative attack of ferricion on aluminum and copper (R. F. Wegman, D. W. Levi, K. M. Adelson andM. J. Bodnar, "The Function of the P2 Etch in Treating Aluminum Alloysfor Adhesive Bonding", Proceedings, 29th National SAMPE Symposium, April3-5, 1984, p. 273.

BRIEF SUMMARY OF THE INVENTION

This invention relates to a novel process and composition for etchingaluminum, which involves the use of other soluble non-carcinogenic,non-polluting metal salts as additives to aqueous sulfuric acid etchantfor aluminum to improve the adhesive bonding to aluminum. The presentinvention is based on the discovery that in addition to the ferricsulfate salt disclosed in U.S. Pat. No. 4,212,701, other solublenon-carcinogenic, non-polluting metal salts generally, when incorporatedin an effective amount into an etchant composition consistingessentially of aqueous sulfuric acid, are effective for improving theadhesive bonding to aluminum when the aluminum is etched with such asalt-sulfuric acid solution prior to adhesive bonding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a chart comparing the shear strength of adhesive bondsobtained with the etchants of the present invention and prior artetchants.

FIG. 2 and FIG. 3 set forth graphical comparisons of the stresseddurability of adhesive bonds obtained with etchants of the presentinvention and prior art etchants.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention can be carried out by contactingthe aluminum (which term includes bare or clad aluminum as well asaluminum alloys consisting predominantly of aluminum) with an etchantcomposition consisting essentially of aqueous sulfuric acid and one ormore soluble, non-polluting, non-carcinogenic metal salts other thanferric sulfate. Suitable metal salts include ferric nitrate, ferroussulfate, and magnesium sulfate. The metal salt is employed in an amountwhich is effective to improve the adhesive bonding to aluminum when thealuminum is etched with the salt-sulfuric acid both prior to bonding, ascompared to that similarly obtained in the absence of the metal salt.The amount of metal salt employed can vary widely and depends on theparticular salt utilized. Suitable etchant compositions for carrying outthe process of the present invention contain sulfuric acid, metal saltand water in proportions equivalent to the following:

    ______________________________________                                                         Weight Percent                                               ______________________________________                                        Sulfuric acid 96% sp. gr. 1.84                                                                   25 to 35                                                   metal salt, anhydrous basis                                                                      5% to saturated solution                                   water              50 to 70                                                   ______________________________________                                    

Metal salts containing a metal cation in a higher rather than a loweroxidation state are generally more effective for promoting adhesivebonding of aluminum when employed in the etchant baths of the presentinvention, and hence are preferred. Ferric nitrate is a particularlyeffective metal salt for use in the present invention and is preferablyemployed in the novel etchant compositions containing about

25 to 35 weight percent sulfuric acid 96%,

5 to 15 weight percent ferric nitrate and

50 to 70 weight percent water.

The process of the present invention can be accomplished by contactingthe aluminum with a novel etchant solution for a suitable period over awide range of temperatures, e.g. from 10° C. to 100° C. for about 5 to30 minutes.

The following examples illustrate the process and compositions of thepresent invention.

Example 1 Preparation of P2 Etchant (U.S. Pat. No. 4,212,701)

370 grams of sulfuric acid 96% sp. gr. 1.84 were slowly stirred into 500milliliters of deionized water. 150 grams of ferric sulfate 75% (Fe₂(SO₄)₃ ×H₂ O were then added. The resulting mixture was diluted to avolume of one liter with deionized water and stirred until a completesolution was obtained. The etchant composition thus produced containedapproximately 28.7% by weight 96% sulfuric acid, 11.6% by weight ferricsulfate 75% and 59.7% by weight water.

Example 2 Preparation of P3 Novel Etchant.

The procedure of Example 1 was repeated except that 153 grams of ferroussulfate (FeSO₄. 7H₂ O) were employed in place of the ferric sulfate. Theetchant composition thus produced contained approximately 29.3 weight %sulfuric acid 96%, 12.1 weight % (FeSO₄.7H₂ O=6.6 weight % anhydrousFeSO₄ and 58.6 weight percent water.

Example 3 Preparation of P4 Novel Etchant.

The procedure of Example 1 was repeated except that 66 grams ofanhydrous magnesium sulfate (MgSO₄) were employed in place of the ferricsulfate. The etchant composition thus produced contained approximately30.9 weight % sulfuric acid 96%, 5.5 weight % magnesium sulfate and 63.5weight % water.

Example 4 Preparation of P5 Novel Etchant.

The procedure of Example 1 was repeated except that 222 grams of ferricnitrate (Fe(NO₃)₃.9H₂ O were employed in place of the ferric sulfate.The resulting etchant composition contained 28.6 weight % sulfuric acid96%, 17.2 weight % ferric nitrate (Fe(NO₃)₃.9H₂ O=10.3 weight %anhydrous Fe(NO₃)₃ and 54.2 weight % water.

A control SD etchant solution of aqueous sulfuric acid was prepared insimilar manner except that no metal salt was added in order to study theeffect of sulfuric acid in the absence of a metal salt.

Table 1 sets forth the compositions of the etchant solutions thusprepared.

                  TABLE 1                                                         ______________________________________                                        Materials        P2      P3      P4    P5                                     ______________________________________                                        Sulfuric acid 96%                                                                              370     370     370   370                                    Ferric sulfate 75%                                                                             150                                                          Ferrous sulfate FeSO.sub.4.7H.sub.2 O                                                                          153                                          Magnesium sulfate M.sub.g SO.sub.4                                                                      66                                                  Ferric nitrate Fe(NO.sub.3).sub.3      222                                    Deionized water to make                                                                        1 liter 1 liter 1 liter                                                                             1 liter                                ______________________________________                                    

Etching Procedure

The etchant solutions prepared above were employed to etch finger panelsof bar or clad aluminum 2024T3 of 1/16 inch thickness, which had beencarefully cleaned with acetone to remove dirt, grease and fingerprints.The aluminum 2024T3 had the following composition:

4.5% Cu; 1.5% Mg; 0.6% Mn; 93.4% Al.

The panels were then immersed for 11 minutes in the etchant solutionspreheated to and maintained at 150° F.±5° F. Immediately after removalfrom the etching bath. The panels were immersed in tap water for twominutes, sprayed with deionized water at room temperature to rinse offthe tap water, then dried at 140° F. for 35 minutes in an aircirculating oven and cooled to room temperature.

Bonding Procedure

The panels were adhesively bonded to form a single lap joint in eachcase. Prior to etching the panels had been cut and drilled to producethe desired 1×0.5 in. bonding area. For each bonded panel a one inchwide strip of adhesive was laid across the bonding area to insurecomplete coverage of the desired one half inch area. The adhesiveemployed was a thermosetting epoxy film adhesive EA9628NW, marketed byHysol Corp., Olean, NY, which was cured at 250° F. for one hour at 25psi. maintained on the bonded panels in a heated press.

The bonded specimens were tested for wear strength and stresseddurability, as described below.

Shear Strength Tests

The shear strength tests were carried out at temperatures of 73° F.(room temperature) and 140° F. at about 50% relative humidity butotherwise essentially according to the method described in ASTM D1002-72Standard Method of Test For "Strength Properties of Adhesive in Shear byTension Loading (Metal-to-Metal)" to provide control data. Sixreplicates were run in each case.

The test results are set forth in FIG. 1 wherein average values of shearstrengths were employed. The results show that at both room temperatureand at 140° F. the bonds obtained with the P5 etchant containing ferricnitrate are closely similar to those obtained with the P2 etchantcontaining ferric sulfate and significantly superior to the bondsobtained with the P3 and P4 etchants containing ferrous sulfate andmagnesium sulfate, resp. as well as the SD sulfuric acid control etchantcontaining no metal salt. While the results show that the bonds obtainedwith the P3 and P4 etchants possess equal or slightly inferior shearstrength as compared to those obtained with the control SD sulfuric acidetchant, they possess superior stressed durability, as shown below.Experience has shown that the shear strength test is a less reliableprocedure than the stress durability test for discriminating anddetermining the effectiveness of surface treatments for adhesivebonding.

Stressed Durability Tests

Each bonded specimen was placed in a spring-loaded jig and subjected ina chamber to a temperature of 60° C. (140° F.) and condensing humidityessentially according to the method described in 20 ASTMD2919-71,Standard Recommended Practice For Determining Durability of AdhesiveJoints Stressed in Shear by Tension Loading. The loads applied weredetermined by using the shear strength control values at 140° F. Thespecimens were loaded at 50%, 40%, 30% and 20% of the average 140° F.shear strength, using five replicates in each case.

FIGS. 2 and 3 set forth graphically the stressed durability results forthe 2024T3 bare or clad aluminum containing the aluminum clad on acopper base. (Identical results are obtained since the copper base isnot exposed by the etching treatment). These figures show that at thehigher stress levels the sulfuric acid etchants containing

a) ferric sulfate P2, ferric nitrate P5, ferrous sulfate P3 andmagnesium sulfate P4 are more effective than sulfuric acid alone SD;

b) ferric sulfate and ferric nitrate are substantially superior toferrous sulfate and magnesium sulfate, suggesting that salts containingthe metal cation in a higher oxidation state are more effective;

c) ferric nitrate is only slightly less effective than ferric sulfate,suggesting that no further advantage results from the use of a morestrongly oxidizing anion when the salt contains the metal cation in ahigher oxidation state.

Prior to bonding the etched surfaces were examined by use of thescanning electron microscope (SEM). The SEM pictures thus obtainedrevealed that the P2 and P5 etchants containing either ferric sulfate orferric nitrate produced much larger pitting on the surface than the lesseffective etchants P3, P4 and SD. In view of the foregoing bonding andother results it appears that such greater pitting provides an increasedsurface area and sites for mechanical interlocking and thereby promotesadhesive bonding.

The foregoing disclosure and drawings are merely illustrative of theprinciples of this invention and are not to be interpreted in a limitingsense. We wish it to be understood that we do not desire to be limitedto the exact details shown and described because obvious modificationswill occur to a person skilled in the art.

What is claimed is:
 1. A composition for etching aluminum preparatory toadhesive bonding, which consists essentially of about25 to 35 weightpercent sufuric acid 96% 5 to 15 weight percent ferric nitrate 50 to 70weight percent water.
 2. A process for adhesive bonding of aluminumparts, wherein the aluminum parts prior to bonding are subjected to anetching treatment to produce a surface preparatory to epoxy resinadhesive bonding, which comprises contacting the aluminum part with anetchant composition consisting essentially of aqueous sulfuric acid andan effective amount to improve the bond strength of a soluble,non-carcinogenic, non-polluting metal salt selected from the groupconsisting of ferric nitrate, ferrous sulfate, magnesium sulfate ormixtures thereof.
 3. A process according to claim 2, wherein the metalsalt is ferric nitrate.
 4. A process to claim 2, wherein the etchantcomposition consists essentially of about25 to 35 weight percentsulfuric acid 96% 5 percent to a saturated solution of said salt. 50 to70 weight percent water.
 5. A process according to claim 3, wherein theetchant composition consists essentially of about25to 35 weight percentsulfuric acid 96% 5 to 15 weight percent ferric nitrate 50 to 70 weightpercent water.